摘要：21． the boy was walking down the street, the UFO landed.A．While B．After C．Before D．until
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_______ the boy was walking down the street，the UFO landed．
A．Why B．Where C．While D．Whenever
查看习题详情和答案>>________ the boy was walking down the street，the UFO landed．
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A．While
B．After
C．As
D．Before